This invention relates to optics for diffracting x-rays of short wavelength and a method of constructing such optics.
All materials have refractive indices very close to one (1.00) in the x-ray portion of the electromagnetic spectrum. As a result, optic structures and instruments for refraction and specular reflection of x-rays cannot be made from a single material. Some reflection of x-ray waves, however, does occur at the interface of two materials having different refractive indices. If such a pair of materials is formed into a large number of alternating layers, with the layered pairs having substantially the same thickness (or at least the corresponding layers of each pair having substantially the same thickness), the reflectances accumulate if the wavelength L and the angle of incidents i (measured from the plane of the reflective surface) meet the requirements of the Bragg equation: nL=2d sin i, where n is the order of diffraction and is an integral number and d is the layer pair thickness. Such multilayer structures, constructed using conventional sputtering (the ejection of atoms or groups of atoms from a source onto another surface to form a thin layer thereof) or vacuum deposition (producing a surface film of metal on a heated surface, typically in a vacuum, either by decomposition of the vapor of a compound at the work surface, or by direct reaction between the work surface and the vapor, have been utilized for reflecting, focusing, dispersing, etc., electromagnetic waves.
However, with the currently used methods of producing the multilayer structures (sputtering and vacuum deposition), it is difficult to develop layers having a thickness of less than about twelve angstroms and so a certain range of x-ray wavelengths cannot be diffracted. At least, it is difficult, if not impossible, to develop layers having a uniform thickness of twelve angstroms or less. The reason for this is apparently because the materials used to construct the layers tend to clump together when physical vapor deposition methods are utilized. That is, the first few atoms deposited tend to aggregate on the surface to form islands several angstroms thick before they begin to grow laterally. When such clumps or islands finally grow together, their crystal orientations are unlikely to match so that a grain boundary is formed. Such imperfections not only themselves degrade x-ray reflection but they also serve as "chimneys" where one material diffuses into another and this, in turn, further degrades x-ray reflection because the refractive indices of the diffused materials are subsequently closer together.
In addition to the above problems with currently used physical deposition techniques for constructing multilayer devices, sputtering, and especially evaporation, oftentimes tend to blur the interface between layers by driving fast moving vapor of one layer into the adjacent layer of material. Furthermore, physical vapor deposition methods tend to be statistical in nature, i.e., atoms arrive at the surface on which they are being deposited at random times and in random places and usually "freeze" into the surface where they first touch. Physical deposition cannot be stopped at the precise time when enough atoms have been added to complete the last layer to thereby even out the variations in coverage of the layer. As a result, one or more of the top layers will be incomplete and this leads to rough interfaces between layers and variations in layer of thickness, both of which decrease reflectance of electromagnetic waves. Also, random fluctuations (in location and quantity) in arrival of the atoms lead to roughness at the interfaces.
Although the imperfections described above are not a serious problem in multilayer structures having thick layer pairs, they can be fatal to the usefulness of structures whose layers are thinner than about twenty angstroms. An illustration of this problem is shown in FIG. 1 of the drawings, where a good x-ray multilayer reflector, shown at 4, has layers of uniform thickness and smooth interfaces, and a poor x-ray reflector, shown at 8, has layers of nonuniform thickness and rough interfaces. Since very thin layers are needed for diffracting short x-ray wavelengths used, for example, in medicine and nondestructive testing, useful diffracting instruments in these fields cannot be constructed with currently available physical deposition techniques.
Crystals have also been used to diffract electromagnetic radiation, including x-rays, and they present diffracting planes which are more nearly perfect. However, crystals are also usually quite small, cannot be bent or shaped much without fracturing, and the range of layer of thicknesses available is limited (all are too small for soft x-rays). Multilayer structures, of course, if they could be constructed with layers whose interfaces are smooth and whose thicknesses are uniform, would provide a solution to these shortcomings since such structures can be specifically designed with (1) areas of a predetermined size (larger than many crystals), (2) layers having a predetermined thickness, and (3) reflecting surfaces which are shaped in a predetermined way, for example curved to focus reflected radiation. The areas of application for a high quality, controllable surface shape and thin multilayer structure are legion, including x-ray microlithography for the production of integrated circuits, and x-ray focusing optics for use in medical diagnostics, biological research, and nondestructive testing (for example, weld inspection, fatigue, cracks, etc., in metal or metal alloys).